- Assign load / store with shifter op address modes the right itinerary classes.

- For now, loads of [r, r] addressing mode is the same as the
  [r, r lsl/lsr/asr #] variants. ARMBaseInstrInfo::getOperandLatency() should
  identify the former case and reduce the output latency by 1.
- Also identify [r, r << 2] case. This special form of shifter addressing mode
  is "free".

llvm-svn: 117519